Biofouling is the growth of marine organisms on a structure. Biofouling includes a nucleation stage comprising a microbiological slime, and a subsequent growth stage of hard fouling comprising seaweed, barnacles, limpets, mussels, etc. The build-up of biofouling is of concern as it increases vessel hull drag (and associated fuel burn and emissions). There is also environmental concern about transportion and of release of marine invasive species through shedding of fouling (whether accidently or through cleaning). FIG. 1 shows a ship's hull with heavy hard biofouling.
For many years the control of biofouling on vessels was achieved using biocide paints containing copper compounds or tributyltin (TBT) which kill organisms and spores, preventing attachment and growth of a biofilm. TBT paints have now been shown to be environmentally damaging and are being progressively banned and phased out. Some “self-polishing” paints contain different biocide compounds, but these are also under increasing environmental scrutiny.
Fouling release paints (FRPs) use a range of low surface energy polymers (sometimes in combination with micro or nano textures) to make it difficult for organisms to attach securely to a vessel's hull. As the organism grows, the hydrodynamic drag force acting on it overcomes the adhesion strength and the organism is pulled off the hull by the flow of water over it. However, FRPs require fouling to build up to a certain level before it can be shed. As a result, FRPs incur an increased drag penalty relative to a hull with no fouling. FRPs also do not address the issue of transportation of invasive species.
Heavy fouling can be removed by sending divers down with cleaning equipment to detach the fouling from the hull. However, the process is slow and costly. Further, poor visibility under water may cause the divers to damage paint or miss areas, resulting in patchy cleaning. In addition, not all ports allow diver cleaning, as the organisms removed by such cleaning are released and may constitute invasive species.
Recently robotic cleaning devices have been developed to remove fouling from the hull. Most include brushes or scouring pads to remove hard fouling, although such aggressive cleaning approaches can damage paint and initiate corrosion. The robots are typically deployed while the ship is stationary.